Controversy surrounds the US state of Missouri which has just carried out its fourth execution by lethal injection in four months.

Michael Taylor and an accomplice were convicted of kidnapping, raping and stabbing to death 15-year-old schoolgirl Ann Harrison in 1989.

The US Supreme Court denied several last-minute appeals which included claims that the drug used could cause undue suffering.

Last year Missouri was reported to have reduced the number of drugs used in lethal injections from three to one.

“There isn’t enough research. We don’t know enough about how painful these deaths are,” said David Tushaus, Professor of Legal Studies at Missouri Western University.

A pharmacy in Tulsa, Oklahoma – The Apothecary Shoppe – had agreed not to provide a drug for the execution after the death-row inmate’s lawyers alleged it could cause “inhumane pain”. It is not known who supplied the substance that was used, but it is presumed to be a compounding pharmacy.

Faced with shortages, several US states have reportedly been turning to new suppliers or unofficial products since a Danish firm took action to ban its drug from being used for executions.

Lundbeck, which describes itself as a global pharmaceutical company working with brain diseases, said in a July 2011 statement that it adamantly opposed “the distressing misuse of our product in capital punishment”.

Lawyers for Taylor also argued he should have been offered a life sentence, or at least a sentencing by a jury instead of a judge – and said that Missouri should allow all appeals to be exhausted before they proceeded with the execution.

Death penalty opponents and a federal judge have criticised Missouri for putting condemned inmates to death while petitions for a stay are still pending.

Taylor’s execution was the state’s 72nd since the death penalty was reinstated there in the 1970s.

It continued as scheduled after the string of terse denials issued for the high court by Justice Samuel Alito.

Governor Jay Nixon had refused to grant Taylor clemency earlier in the day.

Before the execution, Taylor’s family issued a statement saying he had showed remorse and that life imprisonment would have been sufficient.

As reported in the Kansas City Star, they said Taylor had struggled with guilt for years, and had “dedicated much of his time in prison to the memory of Ann Harrison through his work with hospice, tutoring and mentoring”.

Michael Taylor and co-defendant Roderick Nunley – also sentenced to death – spotted Ann Harrison waiting for a school bus while driving in her neighbourhood in a car they had stolen.

They grabbed the teenager, bundled her into the vehicle, and took her away to a basement where they tied her up.

She was raped by Taylor – he said Nunley also took part, something the co-defendant always denied – and stabbed her to death.

Before Taylor’s execution, Janel Harrison, mother of the victim, also made a public statement, asking for justice for her daughter.

“Regarding Taylor spending the rest of his life in prison… when you commit the ultimate crime, which is murder, then there should be an ultimate penalty. If Ann had been allowed to live Taylor would have received a life sentence,” she said.